This invention relates to spark timing control systems for spark ignited internal combustion engines and particularly to such systems including both a closed loop knock limiting control in which spark timing is varied in response to the output of an engine mounted knock sensor to limit engine knock and an open loop knock anticipating system in which a short duration knock eliminating retard pulse is generated in response to the detection of a knock anticipating event. In such a system including both closed loop and open loop controls of the same spark timing variable, the designer is faced with the problem of the best way in which to combine the closed loop and open loop signals for the maximum benefit from each system.
In a particular system of interest, signals from an engine mounted knock detector are transformed into a series of constant amplitude, variable duration pulses indicative of knock intensity, which pulses are supplied to an integrating operational amplifier having a fixed reference, the output of which is a knock level indicating voltage which determines the amount of retard introduced to an otherwise predetermined spark timing. The operational amplifier is optionally provided with output voltage limiting means to limit the amount of additional retard to prevent the possibility of crossfire in those engines subject thereto.
An engine subject to knock may also, however, be subject in particular to tip-in knock, which is initiated by a sudden opening of the vehicle throttle resulting in a sudden increase in pressure within the engine air inlet passage and which generates an audible knock or ping of very short duration. The result may be that the tip-in knock causes the closed loop system to vainly provide unnecessarily long spark retard after the knock has already ended. Even if the engine is subject to continued knock after the tip-in knock, however, the initial tip-in knock will still be heard even if the system prevents or limits the subsequent continued knock.
Therefore, it has been proposed to sense an engine tip-in event by the rate of change of a parameter indicative of air pressure in the air inlet passage such as that pressure itself or throttle position and retard spark timing by an amount of crank angle sufficient to prevent the tip-in knock for a period which begins immediately upon the tip-in signal and preferably lasts for a duration no longer that 300 milliseconds from the beginning of the tip-in event. It has been found that such a retard pulse can be put in and removed quickly and is effective to suppress the tip-in knock while not being readily perceived by the vehicle operator in any substantial resulting loss of engine power or vehicle acceleration.
It has been suggested that this tip-in knock eliminating system may be combined with the closed loop system described above in a "highest wins" configuration in which the output voltage of the closed loop knock limiter integrating operational amplifier and a predetermined tip-in retard voltage actuated by a one-shot multivibrator of the proper duration are both connected through diodes to a common input of another operational amplifier so that the greatest of these voltages is passed onto the spark timing system. If the integrating operational amplifier is provided with voltage limiter means, the tip-in retard voltage may be predetermined to be the same voltage limit so that the limit is not surpassed during the tip-in retard. In a case where no such limiter is required, the tip-in retard voltage would be set at some voltage representing a large retard so that there would be some stepped tip-in retard even for a heavily knock limited spark timing. This suggested prior art combination is shown in FIG. 4 of the accompanying drawings and is described in greater detail in the description of the preferred embodiment below.
This approach, however, has two major problems. First, the diodes have forward voltage drops which are temperature sensitive so that the actual retard voltage supplied to the second operational amplifier varies with the ambient temperature of the vehicle. It appears that this variation would be significant and unacceptable. The second objection is that it is not desirable for the tip-in retard to be set at a greater value than is absolutely necessary to prevent the tip-in knock. With an engine where it is set substantially greater than necessary, it may be that the retard will result in some perceptable loss of power in the engine whereas the correct, smaller amount of retard would not. Since the "highest wins" configuration requires a large tip-in retard, this is an undesirable characteristic of this configuration.
It is also preferable not to add the close and open loop signals prior to the integrator, since this would result in the open loop signal being integrated and thus delayed in time, whereas immediate control of the retard is desired in the open loop.